1. Field of the Invention
This invention relates to a method of measuring the vibration of a solid object by means of a vibration measuring apparatus utilizing a high molecular weight piezoelectric film as a vibration detecting element in which the noise likely to be caused by sound waves can be prevented. This invention also relates to a measuring device for use with such a method.
2. Description of the Prior Art
It is known that certain materials such as high molecular weight substances, for example, proteins, poly-.gamma.-methylglutamate and the like exhibit piezoelectricity. It is also known that the shaped articles of polyvinyl chloride, polymethylmethacrylate, or other similar high molecular weight polar substances can be subjected to a so-called polarization treatment in which they are subjected to heat treatment under a high voltage static electrical field to produce shaped articles of high molecular weight substances having piezoelectric properties. Particularly, when polyvinylidene fluoride is used as a high molecular weight polar substance and the molecules therein are arranged in such a crystal structure that can easily produce the highest polarization, piezoelectric constants comparable with those of Rochelle salt are obtained. Typical crystal structures of polyvinylidene fluoride include an .alpha.-type structure and a .beta.-type structure, the latter having a zigzag planer crystal structure and hence being likely to form a polar structure. Usually, polyvinylidene fluoride, when melted and extruded, often assumes the .alpha.-type crystal structure but the crystal structure of the extruded product is wholly or partially changed into the .beta.-type crystal structure when the extruded product is stretched at a relatively low temperature (for example, below 130.degree.C for the homopolymer).
Taking axes X.sub.1 and X.sub.2 along the plane of a film and axis X.sub.3 in the direction of the thickness of the film, as shown in FIG. 1 of the drawing, and applying a stress along the direction of the axis X.sub.1 in the film, the polarization produced along the direction of the axis X.sub.3 is represented as d.sub.31. When a tensile stress is applied in the same axis direction as the direction in which the polyvinylidene fluoride is stretched the piezoelectric constant d.sub.31 produced sometimes reaches as high as 1 .times. 10.sup..sup.-6 c.g.s.e.s.u. at the maximum. In addition, the stress applied to non-oriented or biaxially oriented films along any two directions produces a piezoelectric body having a d.sub.31 as high as 5 .times. 10.sup..sup.-8 c.g.s.e.s.u.
Natural high molecular weight substances such as proteins or many polarized film products of high molecular polar substances other than polyvinylidene fluoride such as e.g., polyvinyl fluoride, polyvinylidene chloride, polyvinyl chloride, polymethylmethacrylate or the like have a maximum d.sub.31 of about 10.sup..sup.-8 to 10.sup..sup.-11 c.g.s.e.s.u and these substances with piezoelectric constants of such order can also be used as piezoelectric elements by combining them with amplifiers suitable for the purpose.
For the high molecular weight substances having piezoelectric properties as stated above, various applications as electric-mechanical or mechanical-electric transducer elements can be contemplated just as for Rochelle salt and ceramic piezoelectric materials. Particularly, when they are used as measuring devices for mechanical vibration, films of any desired size can be obtained due to the nature of the high molecular weight compound and, generally, they provide many advantages such as a high impact strength as compared with inorganic piezoelectric materials, a lower Young's modulus which scarcely interferes with the mechanical vibration when they are used as thin films contacting the object to be measured over a broad area, no selectivity for a particular wave length since they do not have a substantial resonance point, etc.
When such high molecular weight piezoelectric films are used as measuring devices for mechanical vibration, suitable metal thin films such as of gold, platinum, silver, copper, nickel, iron chromium, aluminum, tin, zinc, or the like are usually deposited on both surfaces of the high molecular weight films as electrodes by way of plating, sputtering, etc. and the piezoelectric element contacted with the surface of an object to be measured by suitable means such as screwing, flange connection, etc. Mechanical vibration is measured by converting the vibration to change in the electric field intensity on the film surface which are caused by the shrinking and stretching of the high molecular weight piezoelectric films.
It has, however, been found that noises are sometimes generated by environmental sound waves to produce errors in the results obtained measuring the mechanical vibration of a solid material by using a high molecular weight piezoelectric film devices. It is believed that the noises are generated when the high molecular weight film picks up sound waves in the surroundings because of its high sensitivity also to sound waves.